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Yes it is just a formation, there are many words that are not in the dictionary but are accepted. Here is the link for the rules from RAE that say it doesn't matter so modisto is viable for example. Section 3, paragraph b For example "las poetas" would be accepted too.
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Joze♦Dec 7 '11 at 14:58

@Joze Femenine words are present in RAE dictionary, e.g. look for "concejal" and "concejala": they lead to "concejal, la.", but if you look for "dentisto" it says the word doesn't exist. I have never heard of it, and it sounds terrible to me (but modisto is OK). Not all names can have a distint word for femenine gender: the most famous case is "miembro" (A Spanish minister used "miembra" and was widely critized because it wasn't correct") Ngram Viewer gives no use of "dentisto": books.google.com/ngrams/…
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JaviDec 7 '11 at 15:32